Marine Atlas: Maximizing Benefits for Vanuatu

VOYAGE TO THE BOTTOM OF THE SEA: GEOMORPHOLOGY

Vanuatu’s sea floor is rich in physical features that affect the distribution of biodiversity, fishing grounds, deep-sea minerals and even tsunamis and underwater landslides.

The nation’s seascape is as diverse underwater as its landscape above, including towering underwa- ter mountains (seamounts) that attract migratory species from hundreds of kilometres away, and deep-sea canyons that carry nutrient-rich water from the deep ocean to the shallow areas. Geo- morphology (the study and classification of these physical features) reveals both the geological origin of the features as well their shape (morphol- ogy), size, location and slope. The geomorphology of the sea floor influences the way the ocean moves (see also chapter “Go with the flow”) and the distribution of water temperature and salinity (see also chapter “Hotter and higher”). These factors affect the distribution of biological commu- nities, resulting in different biological communities being associated with different types of sea-floor geomorphology. For example, seamounts generally have higher biodiversity and a very different suite of species to the adjacent, deeper abyssal areas. Similarly, different economic resources are often associated with different features. Many fisheries operate on certain features, such as the shelf,

Vanuatu’s waters harbour 18 different geomorphic features, which are presented in this map and as- sociated figures. The distribution of geomorphol- ogy reflects many of the patterns observed in the bathymetry map, as geomorphology is primarily a classification of the shape of the sea-floor features. The main Vanuatu islands are perched on a raised plateau, surrounded by an area of generally narrow shelf, which supports exten- sive coral reefs. Ninety-three canyons incise the slope adjacent to the islands. These canyons are characterized as areas of high biodiversity due to their steep sides featuring rocky slopes, strong currents and enhanced access to food. They also act as a conduit between the deep-sea floor and the shallow shelf areas. By Pacific Island standards, Vanuatu has relative- ly few seamounts, with only 13 found within its EEZ. Seamounts are large—over 1,000 metres high—conical mountains of volcanic origin, while guyots are seamounts with flattened tops (see also chapter “Underwater mountains”). There are also numerous ridges and chains of abyssal mountains rising up from the sea floor. On all these features, areas of steep sea floor (escarpments) are likely to contain hard substrate which, coupled with increased current flow, create ideal habitats for filter-feeding organisms such as sponges and cold-water corals. Vanuatu’s waters also have several deep ocean trenches, reaching depths greater than 6,000 metres. These trenches include the North and South New Hebrides Trenches, which reach over 8,000 metres at their deepest points. These deep ocean trenches are likely to support a suite of unique species.

1999 tsunami

On 26 November 1999, central Vanuatu was struck by a 7.5 magnitude earthquake, generating a tsunami that killed five people and caused major damage to nearshore infrastructure. The tsunami is thought to have been generated by a submarine land- slide in the Selwyn Strait (between Pente- cost and Ambrym), highlighting the impact that these out-of-sight structures can have on life on land. slope or over seamounts, based on where their target species occur. In Vanuatu, important deep- sea snapper is mostly found on outer reef slopes and around seamounts (mainly in depths from 100 to 400 metres; see chapter “Fishing in the dark”). Furthermore, different types of deep-sea mineral deposits are also associated with different features, such as the sea-floor massive sulfide de- posits found along mid-ocean ridges (see chapter “Underwater Wild West”).

0 km

200

400

Ridge

2

Pinnacles

Guyot

Seamounts

Abyssal Hills

Trough

4

km depth

Seamount

Continental Crust - Granite

6

Sediment

Ocean Crust - basalt

Subduction Zone

Sediment Drifts

Upwelling lava

MAXIMIZING BENEFITS FOR VANUATU

SUPPORTING VALUES

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